Electric power cables are used to transmit electric power at a medium or high voltage. Different insulation materials can be used in power cable applications, including paper and oil, mass-impregnated cables and polymeric materials. Extruded power cables comprise normally a conductor and radially surrounding polymeric insulation system comprising at least two semi-conducting layers and one insulation layer comprising polymeric material. Electric power cables may be buried into the ground whereby they are called land cables. The electric power cables may also be buried into a sea bed or they may extend between two fixing points in sea water and cables of this type are called submarine, sea water or underwater power cables. Areas where energy is on the one hand needed and on the other hand produced may be located at a long distance from each other, which increases a need for safe power transfer.
In order to meet the demands for safe power transfer, the insulation systems in the cables need to be of high quality to ensure correct electrical and mechanical behavior during the transmission of electric power. To electrically insulate the conductor, an insulation system including semi-conducting and insulating polymeric layers is arranged to surround the conductor. Unless the power cables are appropriately insulated, significant leakage currents will flow in the radial direction of the cables, from the conductor to the surrounding grounded screen. Such leakage currents give rise to significant power losses, as well as to heating of the electrical insulation. The heating of the insulation can further increase the leakage current due to the reduction of the resistance with the increasing temperature. To avoid power losses and possible thermal runaway, the leakage current should therefore be kept as small and stable as possible.
There are limitations to the length of cable that can be continuously manufactured. Therefore, in order to be able to transfer power over the large distances required, it is necessary to be able to safely and effectively join separate lengths of cable. When a cable has to be joined, or spliced, with another cable, all of the cable layers of one cable must be joined to the corresponding layers of the other cable. The present invention concerns in particular issues related to the cable insulation in such a cable joint.
A common type of electric power cable is a cross-linked polyethylene insulated cable, which is usually called XLPE cable for short. This type of cable has an insulation layer produced by extrusion of a low density polyethylene (LDPE) base polymer comprising an organic peroxide cross-linking agent. The extruded insulation layer is then subjected to high temperature and pressure curing conditions in order to homolytically cleave the organic peroxide, forming free radicals that facilitate cross-linking of the polyethylene, thus forming XLPE.
Cables may be joined using a variety of methods. Land cables are commonly joined using prefabricated joints, which are pre-molded devices to which the cable ends are connected. Submarine cables are commonly joined using sea joints, also known as factory joints or flexible vulcanized joints (FVJ).
When producing a flexible vulcanized joint, first of all the conductor ends are denuded of all external layers, commonly by tapering down the cable insulation system to form a conical shape with the exposed conductor protruding from the top of the cone. The conductors are then electrically and mechanically connected to each other, often by welding, soldering or brazing. Next, the electric insulation system is systematically restored. This is done by first restoring the inner semi-conducting layer by winding an extruded semi-conducting tape around the conductor, followed by melting and curing. Then, the insulation layer is restored by winding an extruded insulating tape around the newly produced inner semi-conducting layer, followed by melting and curing. Finally, the outer semi-conducting layer is restored by winding an extruded semiconducting tape around the newly produced insulation layer, followed by melting and curing. The goal is to recreate the cable in the joint by building it from the inside out and thus restore all layers of the cable in the joint.
It is essential that no impurities are incorporated into the insulation system during the production of the joint, since this could lead to impaired insulating properties, the incidence of stress points, and ultimately joint failure. Therefore, the layers of the insulation system are produced under clean conditions. Each step of winding the extruded tapes is performed in a glove box under a positive pressure of clean air to avoid contaminants. The partly-produced joint is then transferred to a vulcanization tube where it is cured using elevated temperatures under a nitrogen atmosphere.
However, despite the meticulous conditions used, joints prepared in this manner still have a higher leakage current than the extruded XLPE cables and therefore an increased risk for thermal runaway, and ultimately failure. Therefore, there is a need for a process that provides flexible vulcanized joints that have a higher robustness and stability.